Computer systems which are coupled to a large amount of data storage may spend a great deal of CPU and BUS time reading and writing data to and from the storage devices. Data retrieval from disk drives or similar data storage devices tremendously impacts and decreases system performance and throughput.
Accordingly, many systems now include cache memory which can be accessed much more rapidly by the CPU without the inherent delays associated with the mechanical motion and time delay inherent in a disk drive. Thus, if the required data is stored in cache instead of on the disk drive, system performance can be increased tremendously. Cache memory, however, is a finite resource. It is costly and accordingly, must be properly managed to yield its intended benefit.
Prior art systems or methods aimed at managing cache memory include a system disclosed in U.S. Pat. No. 4,489,378 wherein the system prefetches a selected number of records in anticipation that the data will soon be requested. Several problems, however, exist with such a system. For example, the system has no knowledge of what is already stored in cache and thus, the system must make real time decisions as to whether or not a sequential data access is being performed. Further, the system prefetches a selected number of data elements without regard for what is currently in cache. Most importantly, such a system is not efficient when used with a multi-tasking/multi-processor system wherein a storage device controller is handling requests in a multiplexed fashion from many hosts. In such a case, multiplexed sequential data access would appear as a non-sequential or random data requests to the cache manager and the desired data would not be present in the cache due to the non-sequential appearance of the task.
An additional prior art system for controlling or managing cache memory is disclosed in U.S. Pat. No. 4,853,846 wherein a cache memory directory is split up into as many segments as there are processors or hosts which are accessing the memory. Given the associated high cost of cache memory, such a system results in expensive, duplicative cache memories. In addition, such a system is also incapable of handling multi-tasking systems given that the system requires a dedicated cache directory associated with each accessing host or processor.
Further, both referenced prior art systems as well as other similar prior art systems are not capable of allowing the user to establish selectable thresholds or criteria to determine the threshold for determining that a sequential task is indeed in progress, the number of data records to prefetch once a sequential task has been identified, and the size of cache memory to allocate to a given task.